This invention relates to secondary alkaline batteries and, more particularly, to shaped positive and negative electrodes for use therein.
Secondary alkaline batteries are particularly suited for a wide variety of applications ranging from power generation in air-borne systems to use in portable tools and appliances to engine starting and, importantly, to electric vehicle applications, due to the high energy densities which can be achieved. Typical electrode combinations include silver-zinc, silver-cadmium and nickel-zinc.
Nickel-zinc batteries have shown particularly outstanding potential. This potential has however not been commercially realized. Thus, the use of zinc electrodes in secondary batteries has been severely limited by their failure to withstand repeated cycling without an irreversible loss of capacity due to undesirable changes in the electrode structure which occur upon repeated recharge. The difficulty in achieving satisfactory cycle life becomes more pronounced for applications requiring relatively significant depths of discharge.
More particularly, in nickel-zinc battery systems using conventional aqueous solutions, such as potassium hydroxide, as an electrolyte, the zinc material, during discharge, is soluble in the electrolyte to a significant extent. Some of the active zinc material thus tends to enter the electrolyte while the battery system is being discharged and while the system stands in a discharged condition. Upon recharging of the battery system, these zinc species in the electrolyte return to the zinc electrode but not without altering the electrode structure. Typically, the active zinc material tends to migrate from the edges or periphery of the electrode structure and collects in the central regions of the electrode, resulting in an irreversible loss of capacity. It has been found that the zinc migration eventually ceases or at least reaches a plateau at which the migration rate is reduced to a minimal level; but, by the time this occurs, the loss in capacity which has resulted is substantial, often making further battery usage impractical. This phenomenon has been often termed "shape change".
A considerable amount of research has been directed to developing an understanding of the underlying reasons for this so-called "shape change". It has thus, for example, been theorized that the change in the electrode configuration is due to the development of concentration gradients of the soluble zincate ion in the commonly used electrolytes. Accordingly, it has been proposed to reduce the solubility of the zinc active material in the electrolyte by suitable modification of the electrolyte. Further proposed solutions have been directed to enclosing the electrodes in electrolyte-impermeable separators so as to isolate the electrodes from the supply of electrolyte that would normally be present along the edges, top and bottom of the zinc electrodes.
Still other solutions have been directed to providing zinc electrodes of greater strength. Thus, for example, the use of binders such as polytetrafluoroethylene and plastic latexes such as polystyrene, butadiene-styrene and polyvinylchloride have been suggested.
Other proposals have included the use of shaped zinc electrodes. For example, the use of an electrode tapered from a thick top edge to a thinner bottom edge has been suggested. Also, U.S. Pat. No. 3,493,434 to Goodkin discloses two types of shaped zinc electrodes. In one embodiment, the volume density of active material of the electrode varies because the volume upon which the density computation is made includes a depressed central region, i.e., the electrode thickness varies directly with the amount of active material per unit area. In a second embodiment, the actual electrode density of active material is varied and provided with a high density region adjacent the periphery of the electrode and a low density region away therefrom towards the center. In both embodiments, the shaped electrode configurations illustrated vary from a minimum amount of active material at the central region and progressively increases towards the periphery of the electrode, terminating at or adjacent the edges.
Despite all of this effort, the commercial potential of nickel-zinc batteries has simply not been realized. A considerable amount of research effort is however being carried out in an effort to provide batteries of this type which combine satisfactory cycle life without sacrificing the high energy densities that such systems are capable of providing. Stated another way, the solutions employed to counteract the "shape change" phenomenon have been at the expense of the energy densities that can be realized, with the resulting battery systems providing little incentive for commercial usage in view of the cost and performance characteristics achieved with other types of available battery systems.
It is accordingly a primary object of the present invention to provide electrode configurations for secondary alkaline battery systems which are capable of providing increased cycle life without any significant sacrifice in the energy densities realized.
A further and more specific object provides flat negative electrodes having flared edges suitably proportioned to compensate for shape change occurring during operation of the battery.
Yet another more specific object of the present invention lies in the provision of positive electrodes compatibly shaped for utilization with the flared edge negative electrodes to allow achievement of relatively high energy densities.
A still further object is to provide a secondary alkaline system capable of achieving increased cycle life by employing optimum ratios of active negative to positive material at selected areas of the electrodes.
Yet another object of this invention provides electrode configurations that minimize the possibility of separator rupture.